Frequency synthesizers are electronic circuits that generate a range of frequencies from a single reference frequency. They are widely used in many different types of devices to tune a transmitter or a receiver to a selected signal frequency of interest, from mobile phones and other communication devices, to televisions and radios, to GPS systems.
Frequency synthesizers commonly use a Phase-Locked Loop (PLL) to provide feedback for a voltage controlled oscillator (VCO) that generates the range of frequencies. A representative prior art system is shown in FIG. 1. A master oscillator 10 is used to generate a reference frequency that is very stable and contains as little noise as possible. A fixed counter 12 divides the reference frequency. It provides an input to phase detector and charge pump 14 which compares the phases of two signals and generates an output signal proportional to the difference between them. This signal is filtered by loop filter 16 and used to control VCO 18 to generate an output frequency. A feedback loop through programmable counter 20 provides the second input to phase detector and charge pump 14 that is compared with the output of fixed counter 12. The feedback loop uses programmable counter 20 to generate a “divided by n” version of the signal output fOUT from VCO 18 which is compared to fREF. Thus, the output frequency fOUT is locked such that if the output frequency drifts in phase relative to the reference frequency fREF, the output of phase detector and charge pump 14 will increase, driving the frequency in the opposite direction and reducing the error.
Communication systems are vulnerable to large in-band (or near in-band) interferers near the signal frequency of interest which can be magnified by poor close-in (also known as “1-over-f” or flicker) phase noise, generally dominated by local oscillator (LO) and sampling clock frequency sources. A significant contributor to close-in phase noise (within the frequency source, i.e., the PLL hardware) is the frequency synthesizer loop controller's charge pump circuitry.
Thus, a need exists for a PLL frequency synthesizer that reduces the close-in phase noise contribution of the PLL charge pump.